Shattered: Forensic Glass Analysis

What is Glass?An inorganic product of fusion which has cooled to a rigid condition without crystallizing Uniform amorphous solidNo specific m.p.Softens over a temperature range

'Ordinary' sheet glass -made by passing the molten glass through rollers; process gives an almost flat finish but with distortions. -glass is used in glazing greenhouses and garden sheds where the visual distortions do not matter. Float glass (plate) Float glass gets its name from the method of production used to manufacture it. -molten glass is 'floated' onto a bed of molten tin-produces a glass which is flat and distortion free-process imparts a fluorescence to the glass [UV light]

Rolled GlassFloat GlassFloat Glass Process

Toughened (Safety glass) -glass is produced by applying a special treatment to ordinary float glass after it has been cut to size and finished. -treatment involves heating the glass so that it begins to soften (about 620 C) and then rapidly cooling it. -produces a glass which, if broken, breaks into small pieces without sharp edges.Laminated glass -glass is made up of a sandwich of two or more sheets of glass (or plastic), bonded together by a flexible, normally transparent material. -if cracked or broken, the flexible material is designed to hold the glass fragments in place.

Whats in Glass? Network Components-Formers:SiO2, B2O3, P2O5, GeO2, V2O5, As2O3, Sb2O5FluxesSofteners [lowers melting point]:Na2O, K2O, LiO, Al2O3, B2O3, Cs2OStabilizersChemical/Corrosion Resistance:CaO, MgO, Al2O3, PbO, SrO, BaO, ZnO, ZrO

Major Types and UsesSoda lime silicate glassSiO2 + Na2O / K2O + CaO / Al2O3 / MgOFlat glass, container glass, electric light bulbsBorosilicate glass> 5% B2O3 (replaces Na2O)Lab glassware, thermometers, cookware, sealed-beam headlights

Aluminosilicate glassHigher percentage of aluminumhigher temperatures than borosilicateAluminoborosilicate 50:50 Al/BLabware, cookware, and glass fibersLead alkali silicate (leaded glass)Up to 80% PbOhigh refractive indexCrystal tableware,

Different glasses-domestic windows, car headlamps, light bulb glass-are produced differently from different chemistries.

Forensic Individualization of GlassFlat GlassSoda lime silicate-Rolling or FloatingTempered: Rapid CoolingAdds StrengthDices when brokenAutomotive windows & security windowsFloat Glass floresces when excited at 254 nm.

Coated: Surface modificationMirrorsLaminated: Sandwiched around plasticAutomotive windshieldsHeadlights: often borosilicateLight bulbs: soda lime glassHeat absorbing/ UV filteringtintingEyeglasses: prescription lenses/photosensitive

Container GlassLower magnesium, higher sodiumClear vs. greenish (window)Glass FibersFiberglass insulationAlumino-borosilicateBinder (red or yellow) to hold fibers in bundles

Forensic Analysis of GlassFundamental Question: Is it Glass?Isotropic vs. AnisotropicGlass: Isotropicdark when rotated under crossed polarizersCrystalline solids and plastics: AnisotropicChange retardation when rotated under crossed polarizersHardnessSolubility

Comparison of Physical CharacteristicsColor Flatness ThicknessFluorescenceFloat Glass254 nm

Physical PropertiesDensitySink-Float MethodCHBr3, C2H2Br4, Sodium PolytungstateDensity Range: 2.465-2.540 g/cm3

When 2 substances have different RI, light passing through them produces a Becke Line

A Becke line is a band or rim of light visible along a grain/crystal boundary in plane-polarized light.

Glass analysis is often performed using our oldfriend the PLM

-the goal of glass analysisby PLM is to individualize an evidence glass fragment to anexemplar from a crime scene[primary or secondary]

Populations of Glass RIs

GRIM II System-automated system for matching glass RI-uses a step-heating stage and a RI liquid that varieswith temperature

GRIM II identifies the RI match by monitoring avideo image of the glass fragment in the liquid-as it is heated/cooled the contrast is measured untila minimum is reached-the match point

Chemical Composition Inductively Coupled Plasma (ICP)SEM/EDSNeutron Activation Analysis X-Ray Fluorescence

How does Glass break?Forensic Fracture Analysis

RadialBegin at a point and radiateoutward from point of impact-initial crack is on the sideopposite the applied force

Concentric

Motion of the projectile through the glass puts tension on the front surface of the glass, causing concentric fractures. -high-speed projectile [a rock] penetrates the glass, it leaves an exit hole larger than it's entrance hole. -helps to determine the direction on impact.

A determination of the direction of force in breaking a window pane:

-direction of the rib marks[stress marks on broken edges of glass that are perpendicular to one side of glass]

For radial fractures (radiating from the center):- the direction of the force is on the same side as the tangential parts of the rib marks.

Bullets and GlassIts possible to determinethe bullet's direction bynoting the side of the cone-shaped hole left by thebullet.-small opening is on theentrance side

Counting Bullet HolesDetermination of the sequence of bullet holes can be made by noting the radial fractures.

Radial fractures caused by the passage of a bullet stop at pre-existing fracture.